Dietary supplementation with 1.5-2.0 g elemental calcium per day during pregnancy has been found to reduce the incidence of hypertensive disorders of pregnancy in several small randomized trials. It is proposed to test this effect in a randomized, double-blind, placebo-controlled trial conducted in five collaborating clinical centers. Calcium or placebo will be administered to 4500 nulliparous patients who begin treatment during weeks 14-22 of gestation. Therapy will be continued until the termination of pregnancy. Primary endpoints of the study are preeclampsia, eclampsia, and HELLP syndrome. Birth weight will be an important secondary outcome. Contracts were awarded to five clinical centers (U. Alabama, Case Western Reserve U., U. New Mexico, Oregon U., U. Tennessee) and to the data coordinating center (The Emmes Corp.) during March and April 1991.